The problem to be addressed relates to fiber optic communication systems used to transport digital data and more specifically to chromatic dispersion in such systems. Chromatic dispersion is the process by which an optical pulse spreads out as it propagates in any medium which does not have absolutely constant speed of light vs. wavelength. An optical fiber is such a medium and devices such as thin film filters or optical amplifiers also contribute to dispersion.
For example, at a 10 GHz data rate, a “1” pulse representing the digital “1” is about 100 ps duration in time or 3 cm long in space. The basics of Fourier analysis show that a laser signal modulated to carry such pulses is no longer a single pure frequency plane wave but is composed of a range of frequencies about 10 GHz on each side of the central optical frequency (for example 197 THz, i.e., from 196.99 to 197.01 THz, which corresponds to a wavelength range of 0.08 nm). If the speed of light in the system varies enough over this frequency range, the pulse shape will be smeared by the time it gets to its destination, which might be 100 km away. As a result, the “1's” are confused with the “0's”, and the data is lost. For one pulse to be confused with the next over a path of 100 km, it is only necessary that the speed of light in the fiber vary by one part in 108 over the 10 ghz bandwidth.
For optical fiber dispersion measurement purposes, the quantity of interest is group delay per unit length, which is the reciprocal of the group velocity of a particular mode. The measured group delay of a signal through an optical fiber exhibits a wavelength dependence due to the various dispersion mechanisms present in the fiber. Group delay at a given wavelength is measured in ps and its slope in ps/nm, or equivalently in terms of the third derivative of phase shift, whose units are ps2. Chromatic dispersion is measured in terms of the rate of change of group delay as a function of wavelength. In other words, it is measured in ps/nm/km, i.e., ps of differential delay per nm of wavelength per km of travel distance. So, total dispersion can be as much as −5000 to +5000 ps/nm.
Dispersion is reversible if elements that have opposite effects on the speed of the light at the various wavelengths are inserted into the pulse path. For example, if red travels more slowly in the fiber than does blue, then an element that slows down the blue in comparison to the red will undo the effects caused by traveling through the fiber. That is, such an element can reverse the dispersion, restore the pulse shape and thus “fix” the pulses.
Dispersion compensation means introducing into the network some optical element, which by providing an equal and opposite dispersion characteristic undoes the dispersion caused by other elements. The problem of dispersion caused by fiber spans or amplifiers or other devices in 10 or 40 Gb/s networks and the need for compensation of these effects is described in A. Willner, OPN Trends, March 2002, p S-16, Optical Soc America, which also discusses methods of tunable dispersion compensation using fiber Bragg gratings.
There are special fibers which have this dispersion compensating property. They are used by periodically placing short lengths of this special fiber in the network to offset the accumulated dispersion. This is not an optimum fix, however. In addition, there is no mechanism to adjust it short of uninstalling it.
Another adjustment mechanism is a special fiber Bragg grating which is designed with a “chirp” and used in a reflection setup with a fiber circulator. The chirp is such that the grating period is altered along the length of the grating. When a pulse is reflected from the grating, the blue light has to travel deeper into the fiber to find its resonant grating and get reflected; than does the red light which is reflected nearer the beginning. The result is that the delay time varies with wavelength and measured dispersion can be artificially introduced. Some tunable versions of the special fiber Bragg grating have been introduced that involve mechanically stretching the fiber.
Methods using coupled Gires-Turnois etalons are discussed in Lunardi et al, J. Lightwave Tech., Vol 20, p.2136, December 2002. And methods using a pair of thin film filters are discussed in M. Jablonski et al, IEEE Phot. Tech. Lett. Vol. 13, p.1188, November 2001.
The merit of a tunable dispersion compensation technique is determined by the range of group delay dispersion, the compensation bandwidth, the smoothness of this in terms of ripple in ps over the compensation bandwidth, and whether the compensation applies to one single channel for a given device or for all the channels in a WDM datastream. Other desiderata include simplicity, small size and low power requirement, ruggedness, and low cost of manufacture.
No known tunable dispersion compensator, whether based on fiber gratings, etalons, waveguides, free space optics or other techniques is as yet fully satisfactory for industry requirements. So, the search for a better tunable dispersion compensator continues.